Precipitation hardenable, corrosion resistant, chromium-nickel stainless steel alloy



. generally improve stainless steel alloys.

United States fPatentO PRECIPITATION HARDENABLE, CORROSION RESISTANT,CHROMIUM-NICKEL STAIN- LESS STEEL ALLOY Norman S. Mott, Westfield, N.J., assignor to Cooper Alloy Corporation, Hillside, N. J., a corporationof New Jersey e No Drawing. Application February 9, 1956 Serial No.564,353

- 9 Claims. (c1. 75-125 This invention relates to stainless steelalloys, and more particularly to a precipitation h'ardenable, corrosionresistant, chromium-nickel stainless steelalloy. r

A most popular stainless steel alloy is a high chromiumnickel steel,especially the so-called 1-88 stainless steel. In general, thisstainless steel is not hardenable, but was found to be hardenable by theaddition of a substantial quantity of beryllium, which is costly. In myPatent 2,635,044, issued April 14, 1953, and entitled HardenableStainless Steel Alloy, I disclosed such-an alloy which is precipitationhardenable while requiring the addition of only a very small percentageof beryllium.

. The primary object of the present invention. is to A more particularobject is to provide a precipitation hardenable stainless steel alloywhich doesnot require the addition of beryllium and which may be made atrelatively low cost. The alloy is desirable in situations inwhichprecipitation hardening is wanted, .to a very high degree, while notrequiring high ductility, and tensile strength.

The new alloy is based on the addition of molybdenum and silicon andcopper. It has been fOund Wh-en galling is not a problem, but greatresistance to abrasion is :necessary, that beryllium is unnecessary andevenrdetrimental to best results. Best hardnessfor high abrasionresistance is found by using as high a percentage of silicon, copper andmolybdenum as will allow casting without cracking. 1

The molybdenum is used in a range of from v3 to 5%, and more preferably3.754-.25%; silicon is used, in a range of 2.5 to 4%, and morepreferably 3.25 to'3.75%; and copper is used in a range of from 2.5 to4%., and more preferably 2.75 to 3.25%. g

A specific example of my improved alloy, with its resulting hardnessand. physical properties and corrosion resistance, are given in thefollowingtable as alloy X-20. The chemical analysis is given inpercentage by weight.

Patented Jan. 13, 1959 "ice Brinell hardness number; WQ means waterquenched;

.- and PH means precipitation hardened after water the precipitationhardening.

' high silicon, and copper.

In the above table TS refers to the tensile strength of a specimen inpounds per square inch. YS means the yield strength in pounds per squareinch at the customary 0.2% ofiset, that is, departure fromproportionality to show that the elastic limitphas been exceeded. Elrefers to the elongation of the specimen in percentage, before rupture.RA refers to the reduction of area in percentage at the time of rupture.

The decimal figures given in the above table for the corrosion tests areI. P. M. figures, meaning the corrosion rate in inches of penetrationper month when immersed in the indicated acid.

Alloy'Xl8 in the above table illustrates hardeningby the addition ofhigh molybdenum and high silicon without copper. Alloy V-4 has copper,but reduced molybdenum. Alloy X-13 has .the amount of silicon greatlyreduced. .Alloy X-2O illustrates the invention, and shows hardening bythe addition of high molybdenum, It should be noted that this X-ZOalloy, although showing good figures for yield strength and tensilestrength, has only small ductility, that is, the percent elongationfigure is only 2%. However, it has very great hardness increase, andtherefore this alloy must be. considered one which is desirable forTable I Test X-18 V-4 X-l3 '-X-'20 138, 847 185, 400 105, 163 150, 000 821 v 2 RA 5. 5 4 1'8. 1 1. 5 Corrosion Tests: a

%LHNO Boiling I 0134 .0183. .110 .10141 50% H2504 80 F. 00009 00016.00000 00000 5% H01 80 F 0203 00408 00000 017 very high hardness withonly low ductility, compared to X-18,V-4. and ;Xl3.

Additional examples are given in the following Table II:

Table II Test X-46 X-26 X-24 X-14 .038 .034 .040 k .046 19. so 20. 0022. 30 19. 37 s. 45 9. 05 9. 4.5 8. 72 .84 .79 .78 .72 3. 79 5. 00 4. 335. 00 3.58 3. 20 3.16 4. 00 2. 86 2. 96 2.88 2. 88

Both molybdenum and silicon must be used, and either will not aloneserve.

Increasing the, molybdenum content to say 5% increases hardness. Whenthe molybdenum content is greatly increased to say 6.50% theprecipitation hardening rises only a little. A molybdenum content oversay 5 or 6% produces very little increase in precipitation hardeningeffect.

Alloys requiring large quantities of molybdenum, say more than 5%, wouldprobably be undesirable commercially because ofhigh cost. Whilemolybdenum is expensive, it .is only moderate in cost compared toberyllium, which is not required at all in any of the alloys. disclosedin this specification.

The effect ofincreasing the silicon content, up to say 4%, is toincrease hardness. When the silicon was further increased there was aresulting gain in hardness but it was found that the specimen crackedbadly, thus showing that a practical limit for the silicon content isabout 4%. 1 s

The effect of the addition ."of copper is illustrated-in the followingtable: l

T able Ill Test it C percent Alloy X-42 shows that the addition ofcopper and silicon in the absence of molybdenum will not induceprecipitation hardening even with a high silicon content. Alloy X-28shows that when the copper is maintained at a constant level an increasein molybdenum and silicon produces an increase in hardness.

The effect of a change'in amount of copper is shown Alloys X-19 and X-20are substantially alike except for an increase in copper from 2.04 to2.96%. It will be noted that this leads to an increase in hardness byprecipitation hardening, from 363 to 415. In these alloys the molybdenumand silicon percentages were at desirable amounts. in copper to a valueof 4%, and the hardness has fallen ofi, thus showing that a value of 4%of copper would be about the maximum percentage that could be tolerated.In alloy X-l4 the molybdenum has been increased from. about 4 to 5%, andthe precipitation hardening has increased to a value of 601. The bigincrease in hardness here shown represents a useful alloy where hardnessis of primary importance despite reduction in ductility.

It may be explained that the increase to 3% copper in alloy X- wasbeneficial, and that the increase to 4% copper in alloy X-l4 wasbeneficial, but with an accompanying increase of molybdenum from 4% to5%. When the copper was increased from 3% to 4% in alloy X-l7, whilekeeping the molybdenum down to 4%, the increase in copper was notbeneficial.

The effect of increasing the chromium content is considered in thefollowing table:

In alloy X-24 (compared to X-23) the chromium content has been raisedfrom about 18 to 22% and the BHN hardness has increased 84 points. Thisshows that an increase in the percentage of chromium produces anincrease in the precipitation hardening of these silicon, molybdenum,copper alloys.

Alloy X-17 shows a further increase In Table V it will be seen that Iincreased chromium from 18 to over 22%. It was unnecessary to carry thisincrease further because it is well known in this work that increasedchromium ordinarily increases hardness, and the experiments in Table Vwere sufficient to show that this general property applies also to thepresent alloys. vIt is also well known in this art that it is notpractical to carry the increase of chromium above 30%, and it is forthat reason that I consider the useable range of chromium to extend upto 30%.

The effect of increasing the amount of nickel is considered in thefollowing table:

In alloy X-48 the nickel content has been increased from 8.45 to 12%,and the hardness has decreased by 138 points. This shows that increasingthe nickel in an alloy of this type reduces the precipitation hardeningefiect.

In all of the foregoing analyses, the iron content is not included, butit is, of course, understood that the balance is iron, subject to thepresence of small amounts of impurities incidental to the usual meltingpractices whendealing with ferrous metals. To cover this situation I maystate that in addition to the elements named in the analyses, theremainder is substantially all iron.

The maximum carbon content should be no higher than, say, 0.08%.

It will be understood that the alloys are fully resistant to salt spray.Indeed, the acid tests shown in Table I are much more severe than a saltspray test.

The alloys are weldable by using welding rods of the same generalcomposition as the alloy being welded.

The results of the foregoing tables of tests may be summarized with thepercentages rounded off as follows.

The desired result may be obtained by the addition of molybdenum in arange of from 3 to 5%, silicon in a range of 2.5 to 4%; copper in arange of from 2.5 to 4%. I have further found that increasing the amountof chromium produces an increase in hardness and that the reverse istrue for nickel, that is, an increase in nickel decreases hardness. Ingeneral, when dealing with the alloys described in this specification,the yield and tensile strengths increase with the hardness, although notnecessarily in linear proportion.

The present application is a continuation-in-part, and largely adivision of my copending but since abandoned parent application SerialNo. 490,698, filed February 25, 1955, and entitled PrecipitationHardenable, Corrosion Resistant, Chromium Nickel Stainless Steel Alloy.That application disclosed what is referred to commercially by theassignee of these applications as its PH-55 series of alloys, divisibleinto three alloys designated as PH-55A, PH-55B," and PH-65C.

The PH-SSA alloy is characterized by high strength and high hardnesswith fair ductility, and is intended for erosion and abrasion resistanceor for stressed parts in corrosive applications. It is an alloy whichanswers an objective decided upon by the stainless steel castingindustry, expressing the need for a comparatively hard stainless steelalloy having a fair amount of ductility for corrosion-erosion resistancewith a minimum of 5% elongation, at 350 BHN hardness. The corrosionresistance was desired to be about equal to that of a CF-8M alloyof theAlloy Casting Institute or A. C. I

(which has 0.08% carbon maximum, 18% chromium, 8% nickel, and 3%molybdenum).

The PH-SSB alloy is a" ductile alloy characterized by high strength andmedium hardness, and intended for shock resistance and high stresses incorrosive applications. It answers an industry-decided objectiveexpressing the need for a high strength ductile stainlesssteel alloy forstructural purposes, with a tensile strength as near as possible totwice that of a cast alloy of the A. C. I. known as CF-8 (which has0.08% maximum carbon, 18% chromium, and 8%. nickel, and a tensilestrength of 77,000 p. s. i. average). In this industry-objective theyield strength wasto be over 100,000 p. s. i., the elongation was to beover and the corrosion resistance was to be as good as that of the CF-8.High hardness was not too important.

The PH-55C alloy is characterized by very high hardness and lowductility for use in non-stressed, corrosion resisting parts. It answersan industry-decided objective expressing the need for an extremely hardstainless steel alloy having high abrasion resistance, and not requiringductility. It is for parts not subjected to shock or strain.

The present application is directed to the PH-SSC alloy. The PH-55A andPH-SSB alloys are disclosed in companion applications Serial Nos.564,350 and 564,352 which, like the present application, have beendivided from my aforesaid parent application Serial No. 490,698, butwhich technically may be considered to be continuations-in-part ratherthan true divisional applications, because of added examples.

A broad range of composition which will effectuate the invention is asfollows:

Percent C Under .08 Cr 19-30 Ni 8-10 Mo 3-5 Si 2.5-4 Cu 2.5-4

The preferred range is narrower than the broad range given above, and apreferred or narrowed range may be given as follows:

Still more preferably the carbon content is kept under 0.05%. Thechromium may range up to 20.5% as well as 20%, and the nickel may rangedown to 8.5% as well as 9%.

It is believed that the composition and behavior of my improvedhardenable stainless steel alloy, as well as the advantages thereof,will be apparent from the foregoing detailed description. The new alloyis low in cost and high in corrosion resistance. It is soft enough inthe quench annealed condition to be machinable, and may be precipitationhardened by a comparatively low temperature heat treatment. The alloy isresistant to salt spray and acids. The alloy has the high chromium andnickel content of a regular 18-8 type of stainless steel, and thereforeretains the advantages of that type of stainless steel.

It will be apparent that while I have set forth specific examples of myimproved alloy, changes may be made without departing from the scope ofthe invention, as sought to be defined in the following claims. In theclaims the term non-stressed parts is not intended to mean zero stress.It is intended to mean parts not subjected to shock or strain, as thoseterms are commonly understood in this industry. It means a tensile loadso low that ductility is not an important consideration.

I-claim; I

1..A-low cost precipitation hardenable alloy of very highhardness andvery highabrasion resistance for use inv non-stressed partsnot requiringductility, said. alloy consisting essentiallyofa chromium-nickelstainless steel of the type known generally as 18 and 8, havingaddedtheretov molybdenum, silicon and copper, the molybdenum ranging from- 3%to 5.0%, the silicon ranging from 2.5% to 4%, and the copper rangingfrom 2.5 to 4%, the carbon content being less than 0.08%, said alloybeing free of beryllium,.the said alloy being adapted to beprecipitation hardened to a' very high degree of hardness by acomparatively low temperature heat treatment. A

2. A low cost precipitation hardenable alloy of very high hardness andvery high abrasion resistance for use in non-stressed parts notrequiring ductility, said alloy consisting essentially of achromium-nickel stainless steel of the type known generally as 18 and 8,having added thereto molybdenum, silicon and copper, the molybdenumranging from 3.75% to 4.25%, the silicon ranging from 3.25% to 3.75%,and the copper ranging from 2.75% to 3.25%, the carbon content beingless than 0.08%, said alloy being free of beryllium, the said alloybeing adapted to be precipitation hardened to a very high degree ofhardness by a comparatively low temperature heat treatment.

3. A precipitation hardenable alloy of the general type known as 18 and8 stainless steel, said alloy having a range of from 19% to 30% chromiumand 8% to 10% nickel, said alloy having added thereto molybdenum in arange of from 3% to 5.0%, silicon in a range of from 2.5% to 4%, andcopper in a range of from 2.5 to 4%, the remainder being essentiallyiron with a carbon contentnot exceeding about 0.08%, said alloy beingfree of beryllium, and being adapted to be precipitation hardened to avery high degree of hardness by a comparatively low temperature heattreatment.

4. A precipitation hardenable alloy of the general type known as 18 and8 stainless steel, said alloy having a range of from 19.5% to 20.5%chromium and 8.5 to 10% nickel, said alloy having added theretomolybdenum in a range of from 3.75% to 4.25%, silicon in a range of from3.25% to 3.75%, and copper in a range of from 2.75 to 3.25% theremainder being essentially iron with a carbon content not exceedingabout 0.08%, said alloy being free of beryllium, and being adapted to beprecipitation hardened to a very high degree of hardness by acomparatively low temperature heat treatment.

5. A precipitation hardenable alloy having approximately the followingchemical analysis: carbon 0.032%; chromimum 20.40%, nickel 8.95%;manganese 0.79%; molybdenum 4.26%; silicon 3.46%; copper 2.96%; and thebalance of the alloy being substantially all iron, the said alloy whenhardened being characterized by very high hardness and abrasionresistance for use in parts not requiring ductility.

6. A precipitation hardenable alloy having approximately the followingchemical analysis: carbon 0.038%; chromium 19.80%; nickel 8.45%;manganese 0.84%; molybdenum 3.79%; silicon 3.58%; copper 2.86%; and thebalance of the alloy being substantially all iron, the said alloy whenhardened being characterized by very high hardness and abrasionresistance for use in parts not requiring ductility.

7. A precipitation hardenable alloy having approximately the followingchemical analysis: carbon 0.034%; chromium 20%; nickel 9.05; manganese0.79%; molybdenum 5.00%; silicon 3.20%; copper 2.96%; and the balance ofthe alloy being substantially all iron, the said alloy when hardenedbeing characterized by very high hardness and abrasion resistance foruse in parts not requiring ductility.

8. A precipitation hardenable alloy having approximately the followingchemical analysis: carbon 0.040%;

7 chromium 22.30%; .nickel 9.45%; manganese 0.78%; molybdenum 4.33%;silicon 3.16%; copper 2.88%; and the balance of the alloy beingsubstantially all iron, the

said alloy when hardened being characterized by very high hardness andabrasion resistance for use in parts not requiring ductility.

9. A precipitation hardenable alloy having approximately the followingchemical analysis: carbon 0.046%; chromium 19.37%; nickel 8.72%;manganese 0.72%; molybdenum 5%; silicon 4%; copper 2.88%; and thebalance of the alloy being substantially all iron, the said alloy whenhardened being characterized by very high hardness and abrasionresistance for use in parts not requiring ductility.

References Cited in the file of this patent UNITED STATES PATENTS2,635,044 Mott Apr. 14, 1953 FOREIGN PATENTS 51,924 France May 25, 1943(Addition to No. 874,676) 866,685 France Aug. 25, 1941 OTHER REFERENCESMetals Handbook, 1954 Supplement, pages 3441. Pub. by the AmericanSociety for Metals, Cleveland, Ohio.

1. A LOW COST PRECIPITATION HARDENABLE ALLOY OF VERY HIGH HARDNESS ANDVERY HIGH ABRASION RESISTANCE FOR USE IN NON-STRESSED PARTS NOTREQUIRING DUCTILITY, SAID ALLOY CONSISTING ESSENTIALLY OF ACHROMIUM-NICKEL STAINLESS STEEL OF THE TYPE KNOWN GENERALLY AS 18 AND 8,HAVING ADDED THERETO MOLYBDENUM, SILICON AND COPPER, THE MOLYBDENUMRANGING FROM 3% TO 5.0%, THE SILICON RANGING FROM 2.5% TO 4%, AND THECOPPER RANGING FROM 2.5 TO 4%, THE CARBON CONTENT BEING LESS THAN 0.08%,SAID ALLOY BEING FREE OF BERYLLIUM, THE SAID ALLOY BEING ADAPTED TO BEPRECIPITATION HARDENED TO A VERY HIGH DEGREE OF HARDNESS BY ACOMPARATIVELY LOW TEMPERATURE HEAT TREATMENT.